The invention relates to a feed system for a rotating cutting tool in accordance with the preamble of claim 1.
Feed systems of this kind are used in the series manufacture of case components, for instance of engine/transmission cases. The respective machining stations, for example transfer machines, fine-boring units, special machines, horizontal boring machines or machining centers are equipped with a feed system for boring tools in which the cutting tool is coupled with a work spindle of the machine tool via the feed system.
The feed system permits to compensate defects occurring during manufacture which may occur, for instance, due to the wear of cutting edges, cutting edge tolerance, adjusting errors or dimensional variations, caused by temperature changes of the machine, etc. Moreover the feed systems permit to form cylinder bores having most narrow tolerances or having bores the radius of which is variable in response to the boring depth (e.g. forming of chamfers, radii, recesses, convex, concave or tapered circumferential walls).
In the known system the cutting tools may be formed on different tool holding fixtures, for instance boring bars, socalled eccentric spindles or feeding heads.
In FIG. 1, which is referred to already in this place, a known feed system distributed by the applicant by the name xe2x80x9ceccentric spindlexe2x80x9d is shown. In this feed system a tool holding fixture 2 is arranged coaxially at an eccentric spindles 4 which, in turn, is rotatably supported in a work spindle 6. The axis of the eccentric spindle 4 is offset vis-à-vis the axis of the work spindle 6 by the measure e. I.e. by rotating the eccentric spindle 4 a cutting tool (not represented) held in the tool holding fixture 2 can be radially offset vis-à-vis the axis of the work spindle 6 and thus a feed motion can be performed.
In the known feed system the adjusting movement of the cutting edge with respect to the work spindle 6 is effected by means of a comparatively complex drive system the principal components of which are briefly described hereinafter.
The known drive system includes a servomotor 8 the output shaft 10 of which is connected to a circulating ball spindle indicated by the reference numeral 12 in FIG. 1. By the circulating ball spindle 12 the rotation of the servomotor 8 is converted into an axial movement by which an adjusting slide 14 is movable in axial direction, i.e. in parallel to the axis of the work spindle 6, in response to the control. In the adjusting slide 14 a connecting member 16 coupled to a connecting bar 18 is rotatably supported. The connecting bar 18 is guided to be axially movable in the work spindle 6. Between the end portion of the connecting bar 18 on the side of the tool holding fixture and the adjacent end portion of the eccentric spindle 4 guided in the work spindle 6 there is disposed a lift-off rotary transformer 20 via which the axial movement of the connecting bar 18 can be converted into a rotation and thus a rotation of the eccentric spindle 4 with respect to the work spindle 6 can be realized.
For the adjustment the servomotor 8 is appropriately driven so that the adjusting slide 14 is moved free from play along its axial guide via the circulating ball spindle 12 and thus the connecting bar 18 immerses into the work spindle 6. The axial movement of the connecting bar is converted into a rotation of the eccentric spindle 4 by the lift-off rotary transformer 20 so that, in response to the control of the step motor 8, the eccentric spindle 4 is adjusted so that the tool cutting edge is adjustable with respect to the work spindle 6
The work spindle 6 is driven via a pulley 22 indicated by a dot-dash line, so that the cutting tool held in the tool holding fixture 2xe2x80x94for instance a boring toolxe2x80x94is rotated. As the components required for adjusting the cutting edge are substantially supported in the work spindle 6 or are connected with the same, considerable efforts have to be made to ensure the desired transmission free from play of the adjusting movement of the servomotor 8 to the eccentric spindle 4.
The system represented in FIG. 1 moreover has a very complex structure, because a multiple conversion of tranalatory movements into rotary movements is required. The moved masses are relatively large in the case of this feed system so that also the support of the work spindle and the motors required for the drive of the work spindle 6 have to be adequately designed.
In contrast, the invention is based on the object of furnishing a feed system for a rotating cutting tool, wherein accurate feed is possible at minimum expenditure in terms of device technology.
This object is achieved by a feed system comprising the features of claim 1.
It is an essential feature of the invention that an adjusting means effecting the feed motion is driven via an internal spindle which is adapted to be driven via a separate feed motor alternatively synchronously with the spindle or at a predetermined difference in speed with respect to the speed of the spindle. In the first case, i.e. when the internal spindle and the spindle are driven at the same speed, no feed motion of the adjusting means is effected, because a transmission element of the adjusting means maintains its relative position with respect to a control surface of the adjusting means. In the latter case, i.e. when the spindle and the internal spindle are driven at different speeds, a feed motion is effected, because the control surface is moved with respect to the transmission element as long as the difference in speed is maintained. As soon as the tool holding fixture supporting the cutting tool is moved into the desired position, the speed of the internal spindle is again brought to the speed of the spindle (or vice versa) so that no further relative movement between the control surface and the transmission element occursxe2x80x94the feeding operation is completed. The control surface is usually formed at the outer circumference of a shaft portion connected to the internal spindle and can be designed, for instance, in the form of an involute or as an eccentric surface with respect to the rotation axis of the spindle.
An essential advantage of the feed system according to the invention resides in the fact that substantially only rotating components are used for effecting the feed motion so that the expenditure in terms of apparatuses can be considerably reduced vis-à-vis the conventional solutions in which translatory motions had to be converted into motions of rotation.
The use of rotation-symmetrical components moreover permits extremely high speeds both for the spindle and for the internal spindle so that high machining outputs can be realized.
By an appropriate selection of the geometries of the control surfaces different feed characteristics can be materialized so that even extremely small dimensional corrections or compensations in the range of 0.001 mm are feasible.
Since the feed motion is effected solely due to a difference in speed between the internal spindle and the spindle, almost the entire range of the control surface can be infinitely exploited by varying the speed. Rapid feed motions can also be realized during the machining operation by selecting appropriate drive systems.
It is especially advantageous when the internal spindle is guided in the spindle at least in sections. In such case the internal spindle can be a hollow shaft so that the coolant/lubricant can be guided through the internal spindle to the cutting edge.
The invention allows for a multiplicity of construction variants for tool holding fixtures. For example, it may be designed as a boring bar having fastened to its outer circumference a bending tool holder, the free end portion of which is radially displaced with the aid of the transmission element which is biased against the control surface.
As an alternative, the tool holding fixture may, for example, be formed as a feeding head, wherein a feeding head member is guided in radial displacement on a guiding head member mounted on the spindle. In this variant, an end portion of the internal spindle, which supports the control surface, may be taken into contact with the transmission element received in the feeding head member.
In this variant it is preferred if feeding head member and guiding head member are connected to each other through parallel springs.
Manufacture of the control surface may be simplified by providing the internal spindle with a control member on which the control surface is formed, so that merely the control member must be subjected to superfinish processing in order to form the control surface.
The control surface may be curved in the form of an involute, orxe2x80x94as already mentioned abovexe2x80x94have the form of an eccentric (with respect to the spindle). It is, of course, also possible to realize other control surface geometries having various gradients depending on the rotational angle.
In addition to the radially adjustable cutting edge, the tool holding fixture is adapted to hold a further premachining tool which is preferably not adjustable by the feed system according to the invention.
Each of the spindle and the internal spindle can be driven via a belt drive orxe2x80x94as an alternativexe2x80x94can be directly coupled to the rotor of an electric motor, for instance a D.C. motor, a threephase motor including a rotary frequency converter (for instance a threephase asynchronous motor).
Principally an analog or digital drive technology can be used for controlling the motors.
Other advantageous developments of the invention constitute the subject matter of the further subclaims.